Added javascript for tags in custom.html

docs/_includes/head/custom.html

 <!-- start custom head snippets -->
 <link rel="stylesheet" type="text/css" media="screen" href="{{'/assets/css/custom.css' | relative_url}}">
 
+{% include vendors/google-tag-manager.html %}
 <!-- end custom head snippets -->
\ No newline at end of file

docs/_includes/vendors/google-tag-manager.html

-
+<!-- Google Tag Manager -->
 <script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
 new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
 j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
 'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
 })(window,document,'script','dataLayer','GTM-T72KG6N');</script>
-
-End Google Tag Manager -->
+<!-- End Google Tag Manager -->
\ No newline at end of file